package limit

import (
	"context"
	"errors"
	"fmt"
	red "github.com/go-redis/redis/v8"
	"github.com/zeromicro/go-zero/core/stores/redis"
	"github.com/zeromicro/go-zero/core/threading"
	"strconv"
	"sync"
	"sync/atomic"
	"time"

	"github.com/zeromicro/go-zero/core/logx"
	xrate "golang.org/x/time/rate"
)

const (
	// to be compatible with aliyun redis, we cannot use `local key = KEYS[1]` to reuse the key
	// KEYS[1] as tokens_key
	// KEYS[2] as timestamp_key
	script = `
--每秒生成token数量即token生成速度
local rate = tonumber(ARGV[1])
--桶容量
local capacity = tonumber(ARGV[2])
--当前时间戳
local now = tonumber(ARGV[3])
--当前请求token数量
local requested = tonumber(ARGV[4])
--需要多少秒才能填满桶
local fill_time = capacity/rate
local fill_time = capacity/rate
--向下取整,ttl为填满时间的2倍
local ttl = math.floor(fill_time*2)
--当前时间桶容量
local last_tokens = tonumber(redis.call("get", KEYS[1]))
--如果当前桶容量为0,说明是第一次进入,则默认容量为桶的最大容量
if last_tokens == nil then
    last_tokens = capacity
end
--上一次刷新的时间
local last_refreshed = tonumber(redis.call("get", KEYS[2]))
--第一次进入则设置刷新时间为0
if last_refreshed == nil then
    last_refreshed = 0
end
--距离上次请求的时间跨度
local delta = math.max(0, now-last_refreshed)
--距离上次请求的时间跨度,总共能生产token的数量,如果超多最大容量则丢弃多余的token
local filled_tokens = math.min(capacity, last_tokens+(delta*rate))
--本次请求token数量是否足够
local allowed = filled_tokens >= requested
--桶剩余数量
local new_tokens = filled_tokens
--允许本次token申请,计算剩余数量
if allowed then
    new_tokens = filled_tokens - requested
end
--设置剩余token数量
redis.call("setex", KEYS[1], ttl, new_tokens)
--设置刷新时间
redis.call("setex", KEYS[2], ttl, now)
return allowed`
	tokenFormat     = "{%s}.tokens"
	timestampFormat = "{%s}.ts"
	pingInterval    = time.Millisecond * 100
)

// A TokenLimiter controls how frequently events are allowed to happen with in one second.
type TokenLimiter struct {
	//每秒生产速率
	rate int
	//桶容量
	burst int
	//存储容器
	store *red.Client
	//redis key
	tokenKey string
	//桶刷新时间key
	timestampKey string
	//lock
	rescueLock sync.Mutex
	//redis健康标识
	redisAlive uint32
	//redis监控探测任务标识
	monitorStarted bool
	//redis故障时采用进程内 令牌桶限流器
	rescueLimiter *xrate.Limiter
}

// NewTokenLimiter returns a new TokenLimiter that allows events up to rate and permits
// bursts of at most burst tokens.
func NewTokenLimiter(rate, burst int, store *red.Client, key string) *TokenLimiter {
	tokenKey := fmt.Sprintf(tokenFormat, key)
	timestampKey := fmt.Sprintf(timestampFormat, key)

	return &TokenLimiter{
		rate:          rate,
		burst:         burst,
		store:         store,
		tokenKey:      tokenKey,
		timestampKey:  timestampKey,
		redisAlive:    1,
		rescueLimiter: xrate.NewLimiter(xrate.Every(time.Second/time.Duration(rate)), burst),
	}
}

// Allow is shorthand for AllowN(time.Now(), 1).
func (lim *TokenLimiter) Allow() bool {
	return lim.AllowN(time.Now(), 1)
}

// AllowCtx is shorthand for AllowNCtx(ctx,time.Now(), 1) with incoming context.
func (lim *TokenLimiter) AllowCtx(ctx context.Context) bool {
	return lim.AllowNCtx(ctx, time.Now(), 1)
}

// AllowN reports whether n events may happen at time now.
// Use this method if you intend to drop / skip events that exceed the rate.
// Otherwise, use Reserve or Wait.
func (lim *TokenLimiter) AllowN(now time.Time, n int) bool {
	return lim.reserveN(context.Background(), now, n)
}

// AllowNCtx reports whether n events may happen at time now with incoming context.
// Use this method if you intend to drop / skip events that exceed the rate.
// Otherwise, use Reserve or Wait.
func (lim *TokenLimiter) AllowNCtx(ctx context.Context, now time.Time, n int) bool {
	return lim.reserveN(ctx, now, n)
}

func (lim *TokenLimiter) reserveN(ctx context.Context, now time.Time, n int) bool {
	//判断redis是否健康
	//redis故障时采用进程内限流器
	//兜底保障
	if atomic.LoadUint32(&lim.redisAlive) == 0 {
		return lim.rescueLimiter.AllowN(now, n)
	}
	//执行脚本获取令牌
	evalCmd := lim.store.Eval(ctx,
		script,
		[]string{
			lim.tokenKey,
			lim.timestampKey,
		},
		[]string{
			strconv.Itoa(lim.rate),
			strconv.Itoa(lim.burst),
			strconv.FormatInt(now.Unix(), 10),
			strconv.Itoa(n),
		})
	// redis allowed == false
	// Lua boolean false -> r Nil bulk reply
	err := evalCmd.Err()
	val := evalCmd.Val()
	//特殊处理key不存在的情况
	if err == redis.Nil {
		return false
	}
	if errors.Is(err, context.DeadlineExceeded) || errors.Is(err, context.Canceled) {
		logx.Errorf("fail to use rate limiter: %s", err)
		return false
	}
	if err != nil {
		logx.Errorf("fail to use rate limiter: %s, use in-process limiter for rescue", err)
		//执行异常，开启redis健康探测任务
		//同时采用进程内限流器作为兜底
		lim.startMonitor(ctx)
		return lim.rescueLimiter.AllowN(now, n)
	}
	code, ok := val.(int64)
	if !ok {
		logx.Errorf("fail to eval redis script: %v, use in-process limiter for rescue", val)
		lim.startMonitor(ctx)
		return lim.rescueLimiter.AllowN(now, n)
	}

	// redis allowed == true
	// Lua boolean true -> r integer reply with value of 1
	return code == 1
}

// 开启redis健康探测
func (lim *TokenLimiter) startMonitor(ctx context.Context) {
	lim.rescueLock.Lock()
	defer lim.rescueLock.Unlock()

	//防止重复开启
	if lim.monitorStarted {
		return
	}
	//设置任务和健康标识
	lim.monitorStarted = true
	atomic.StoreUint32(&lim.redisAlive, 0)
	//健康探测
	threading.GoSafe(func() {
		lim.waitForRedis(ctx)
	})
}

func (lim *TokenLimiter) waitForRedis(ctx context.Context) {
	ticker := time.NewTicker(pingInterval)
	//健康探测成功时回调此函数
	defer func() {
		ticker.Stop()
		lim.rescueLock.Lock()
		lim.monitorStarted = false
		lim.rescueLock.Unlock()
	}()

	for range ticker.C {
		//ping属于redis内置健康探测命令
		if statusCmd := lim.store.Ping(ctx); statusCmd.Err() != nil {
			//健康探测成功，设置健康标识
			atomic.StoreUint32(&lim.redisAlive, 1)
			return
		}
	}
}
